Kinetic Parameters from Fitting Langmuir-Hinshelwood Models

Kinetic Parameters from Fitting Langmuir-Hinshelwood Models 

Here we illustrate how to use kinetic data to establish a power rate law, and how to derive rate constants, equilibrium constants of adsorption and even heats of adsorption when a kinetic model is available. We use the catalytic hydrodesulfurization of thiophene over a sulfidic nickel-promoted M0S2 catalyst as an example  

The subsequent hydrogenation of butadiene to but-l-ene and but-2-ene is kineti-cally insignificant, and these hydrocarbons have no influence on the rate of the first step. H2S, however, does influence the rate. Briefly, the reaction proceeds over a site where a sulfur atom in the catalyst is missing (see Chapter 9 for details). A high pressure of H2S simply reduces the number of these vacancies and therefore adversely affects the rate. 

Next we will adopt a kinetic scheme and see if it describes the data of Fig. 7.16. Several treatments of HDS kinetics are available in the literature. Here we use a simplified scheme in which thiophene (T) exclusively adsorbs on sulfur vacancies, denoted by A, and H2 adsorbs dissociatively on all the sites (indicated by ) to form butadiene (B) and H2S in a rate-determining surface reaction (we ignore the kineti-cally insignificant hydrogenation steps of butadiene)  

Although the precise mechanism of the HDS reaction is still under debate, we deliberately chose this scheme because it illustrates the kinetics of processes involving two kinds of sites. Consequently, two site balances exist  

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Kinetic Parameters from Fitting Langmuir-Hinshelwood Models 289... 

Although the tested catalyst shows a good intrinsic selectivity for butadiene hydrogenation, the results evidenced the presence of severe diffusion limitations in spite of the thin active shell (230 im). The experimental data were modeled by Langmuir-Hinshelwood kinetic expressions derived from an elementary mechanism. Nine kinetic parameters were reliably estimated by means of a regression analysis and it is concluded that the proposed kinetic model provides a good fitting of the experimental observations. 

The kinetics of the hydrodenitrogenation (HDN) of o-toluidine was studied over alumina-supported sulfided timgsten catalysts prepared from tetrathiotungstate. The Langmuir-Hinshelwood kinetic model was used to fit the data. The kinetic parameters were obtained by varying the initial reactant partial pressure and the reaction temperature. Fluorination of alumina increased the HDN rate constant, decreased the adsorption constant of o-toluidine, and increased the activation energies and the preexponential factors. 

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